1. Field of the Invention
The present invention relates to an image processing method and an image processing apparatus for applying gradation conversion to each pixel of a displayed image, for the purpose of effectively obtaining a clear image free from noise.
2. Description of Related Art
FIG. 1 is a block diagram showing the configuration of an image processing apparatus of the prior art wherein gradation conversion is applied by changing the conversion characteristic (xcex3 characteristic) of the pixel value of each pixel in order to make the image clear and easy to recognize. In FIG. 1, numeral 51 denotes a frame memory that stores digital video data. Video data is read from the frame memory 51 and loaded on a look-up table (hereinafter abbreviated as LUT) 52 that is a random access memory as required. The LUT 52 applies gradation conversion to the video data according to the conversion characteristic that is inputted from a characteristic input unit 56, and outputs the converted data to a D/A converter 53. The D/A converter 53 converts the input data to analog signal and outputs it to a CRT 54 that serves as display means. Numeral 55 denotes a timing controller that controls this system.
Operation of the image processing apparatus having such a configuration as described above will now be described below. Pixel value of each pixel is sent from the frame memory 51 successively to the LUT 52 in accordance to address data supplied from the timing controller 55. The LUT 52 receives the data (pixel value) before gradation conversion as address input and outputs the data after gradation conversion to the D/A converter 53 as data corresponding to the address. The data after gradation conversion that is outputted from the LUT 52 is converted to analog video signal by the D/A converter 53 and is inputted to the CRT 54 together with a synchronization signal from the timing controller 55, so that an image that has been made clearer through gradation conversion is displayed on the CRT 54.
Ultrasonic diagnosis techniques to diagnose the conditions of living body by using ultrasonic waves have been widely utilized as methods of diagnosis that do not affect the living body. In the ultrasonic diagnosis, ultrasonic wave emitted from an ultrasonic probe is transmitted into the living body so that reflected waves having different characteristics depending on the difference in the tissue are generated and received by the ultrasonic probe, while the received signal is converted to an electrical signal that is used to display an image on a display device, in order to study the configuration, size, position and other characteristics of the organ from the image.
A typical example of the ultrasonic diagnosis is the tomography of heart (echocardiography). In the ultrasonic diagnosis of heart, because the intracardiac region, pericardial region and myocardial region have reflection characteristics different from each other, signals that are obtained have different levels. Specifically, the signal reflected from the myocardial region has the highest level followed by the signal reflected from the pericardial region and that from the intracardiac region in the decreasing order of the signal level. Therefore, to obtain a clear image that is easier for the observer to recognize in the observation of echocardiography requires it to identify which region is represented by the signal and to apply gradation conversion according to the signal level.
In an image processing apparatus, however, because the characteristic of gradation conversion in the LUT 52 is fixed regardless of the position of the pixel of interest, true noise generated in the intracardiac region that should be eliminated and weak signals from the cardiac wall to be detected are equally suppressed in the ultrasonic image of the heart, for example, thereby making it impossible to obtain necessary images. When it is attempted to detect weak signals that are necessary for diagnosis, on the other hand, sufficient effect of noise suppression cannot be obtained.
An object of the invention is to provide an image processing method and an image processing apparatus capable of producing images easy to recognize for the observer, by automatically changing the gradation conversion characteristic on the basis of the statistical distribution of the pixel values of a plurality of pixels located in the vicinity of each pixel in the image.
Another object of the invention is to provide an image processing method and an image processing apparatus capable of applying gradation conversion with the optimum conversion characteristic with a hardware configuration of relatively small scale, by processing the statistical distribution of the amplitude relationship between the pixel values of pixels in the vicinity of the pixel of interest and predetermined threshold.
In the image processing method according to the invention, conversion characteristic for the pixel value of a pixel of interest is adaptively changed on the basis of the statistical distribution of M pixel values of the pixel of interest and Mxe2x88x921 pixels that surround the same and including thereof (or the pixel values of Mxe2x88x921 pixels surrounding and not including the pixel of interest). Therefore, because the conversion characteristic for the pixel value of the pixel of interest is automatically changed, most suitable conversion characteristic can be selected for the region on which each pixel is based, thereby making it possible to produce images easier for the observer to recognize.
For the statistical distribution used as the criterion of changing the conversion characteristic, the result of comparing the magnitude relationship between the pixel values and the predetermined threshold is employed. Thus automatic operation can be easily achieved with a hardware configuration smaller in size compared to the case of using histogram as the statistical distribution, and realtime operation is also made possible.
Such an arrangement is employed as the M pixels including the pixel of interest and Mxe2x88x921 pixels surrounding the same are located in a rectangular area in the image, and that statistical distribution of the M pixels included in the rectangular area is processed. Consequently, the region where the pixel of interest exists can be more easily identified.
When the same threshold value is used for pixels on one scan line in the rectangular area, a plurality of pixel values on the same scan line can be compared with the threshold by means of a single comparator, and therefore the apparatus can be made more compact by reducing the number of comparators.
Also when the same threshold value is used for all pixels located in the rectangular area, all pixel values can be compared with the threshold by means of one comparator, and therefore the apparatus can be made furthermore compact by reducing the number of comparators.
Further, when comparing the pixel values with the threshold, a comparator of simpler configuration can be used to make it possible to simplify the apparatus, by comparing only several most significant bits.
Furthermore, by using a look-up table when changing the conversion characteristic for the pixel of interest, speed of the changing operation can be increased.
The above and further objects and features of the invention will more fully be apparent from the following detailed description with accompanying drawings.